The endocrinology of the adjult testis has been studied by many investigators over a period of several decades. Regulation of the adult testis by pituitary hormones acting through plasma membrane receptors on the Leydig cell, activation of adenyl cyclase with the resultant production of 3', 5'-cyclic adenosine monophosphate (cAMP), and ultimate steroidogenesis has received much attention. In addition, steroidogenic pathways for androgen production have been exhaustively explored. While some studies have inquired into these aspects of the endocrinology of the fetal and neonatal testis, the parameters of Leydig cell regulation and secondarily, pituitary gland function as it relates to the Leydig cell are not nearly as well understood as in the adult. In order to better understand the endocrinology of the fetal and neonatal rat testis and how these organs differ from each other and the adult rat testis, an experimental plan has been devised which will: 1) Quantify prolactin and FSH receptor content during development in the fetal and neonatal rat testis; 2) Determine if the Leydig cell receptors for LH and prolactin in fetal and/or neonatal rat testes are subject to homologous or heterologous regulation, as has been observed in the adult by both in vivo and in vitro culture system techniques; 3) Quantify the percentage of spare LH receptors in the fetal and neonatal rat testis and assess the sensitivity of these organs to stimulation byLH; 4) Assess the coupling of the LH binding to its receptor, cAMP generation, testosterone and 5Alpha-reduced androgen synthesis in the fetal and neonatal rat testis; 5) Determine the responsivity of the fetal pituitary gland to stimulaion by GnRH; 6) Determine if the fetal pituitary gland regulates LH receptors by in vivo regulation of LH and prolactin secretion. By assessing these functions in the fetal and neonatal rat testis and comparing them with the same parameters in the adult testis, we will be able to better understand the overall continuum of male reproductive endocrinology and the events which contribute to attaining reporductive maturity in the male.